Technologies for integrating metal alloys with resins are required in manufacturing industries of a wide variety of parts and members, for instance in aerospace, automobiles, domestic appliances, industrial machinery and the like. Numerous adhesives have been developed to meet these requirements. Various excellent adhesives are known among these adhesives. For instance, adhesives that bring out their functionality at normal temperature, or upon heating, are used to integrally bond a metal alloy and a synthetic resin. This method constitutes a standard bonding technique used at present.
Other bonding methods that do not rely on adhesives have also been studied. Examples of such technologies include, for instance, methods for integrating aluminum alloys with high-strength thermoplastic engineering resins, without any intervening adhesive, for instance through injection of the resin. The present inventors, for example, have proposed a method that involves bonding simultaneously with resin molding, by injection or the like (hereafter, “injection bonding”), wherein a thermoplastic resin such as a polybutylene terephthalate resin (hereafter, “PBT”) or a polyphenylene sulfide resin (hereafter, “PPS”) is injected and bonded with an aluminum alloy (for instance, Patent documents 1 and 2). A bonding technology has also been disclosed (for instance, see Patent document 3) in which holes are provided in the anodized skin formed on the surface of an aluminum material, and a part of molded body of a synthetic resin is forced into the holes, to be bonded thereby to the aluminum material.
The principles of the injection bonding disclosed in Patent documents 1 and 2 are as follows. An aluminum alloy is immersed in a dilute aqueous solution of a water-soluble amine compound, whereupon the aluminum alloy is finely etched by the weak basicity in the aqueous solution. The above immersion results in the formation of ultra-fine irregularities on the aluminum alloy surface, and at the same time, in adsorption of amine compound molecules onto the surface of the aluminum alloy. The aluminum alloy thus treated is inserted into an injection mold, into which a molten thermoplastic resin is then injected at high pressure.
The encounter between the thermoplastic resin and molecules of the amine compound adsorbed to the aluminum alloy surface gives rise to a chemical reaction. This chemical reaction suppresses the physical reaction whereby the thermoplastic resin cools down quickly, crystallizes and solidify when coming into contact with the aluminum alloy that is held at a low mold temperature. Crystallization and solidification of the resin are delayed thereby, and the resin infiltrates into the ultra-fine irregularities on the surface of the aluminum alloy. As a result, the thermoplastic resin does not peel readily off the aluminum alloy surface even when acted upon by an external force. The aluminum alloy and the resin molded article become strongly integrated as a result. In other words, the chemical reaction and the physical reaction exhibit a competing reaction relationship to each other. In this case, the chemical reaction takes precedence, and the result is a strong injection bonding. In practice it is found that PBT and PPS, which can react chemically with amine compounds, are capable of undergoing injection bonding with aluminum alloys. The above injection bonding mechanism has been termed “NMT (acronym of nano-molding technology)” by the inventors.    Patent document 1: WO 03/064150 A1 (aluminum alloy)    Patent document 2: WO 2004/041532 A1 (aluminum alloy)    Patent document 3: WO 2004/055248 A1 (anodized skin)    Patent document 4: Japanese Patent Application Laid-open NO. 2001-225352
Galvanized steel sheets (ordinarily referred to as zinc-plated steel sheets) are used as a construction material that can be used for 10 years or longer without maintenance. The types ordinarily used at present include precoated steel sheets (also referred to as color steel sheets or color iron sheets) in which one, two or three coats are baked onto a galvanized steel sheet. These sheets exhibit good post-workability and corrosion resistance. The range of applications of galvanized steel sheets, as a construction material, could be expanded if the galvanized steel sheets could be strongly integrated with an adherend such as a resin molded article, FRP, metal alloys and the like. For instance, the steel sheet portion of a composite wherein a galvanized steel sheet and a resin molded article or FRP are strongly integrated with each other could be connected to other construction materials by way bolting, welding or the like. An entire composite obtained from a resin molded article or FRP can take on numerous shapes. The weight of the composite as a whole, moreover, can be reduced by increasing the volume ratio of the resin molded article or FRP. Such a composite is thus extremely useful as a construction material. Meanwhile, strong bonding between a galvanized steel sheet and a metal alloy by way of an adhesive, without relying on bolting or welding, would contribute to reducing costs on account of the fewer parts and fewer processes that would be involved.
However, the above-described strong integration between an aluminum alloy and resin molded articles is achieved through adsorption of amine compounds onto the aluminum alloy surface. Therefore, it is unclear whether galvanized steel sheets can be injection-bonded in the same way as in the case of aluminum alloys, inasmuch as galvanized steel sheets have different surface environments, and are subjected to different immersion treatments, vis-à-vis aluminum alloys. No techniques have been proposed for strongly joining galvanized steel sheets and resin molded articles by way of adhesives or through press-fusion bonding. Likewise, no techniques have been proposed for strongly bonding galvanized steel sheets and metal alloys or FRP by way of adhesives.